1. Field of the Invention
This invention relates to a photoelectric conversion device, and particularly to a process for forming a passivation film on a photoelectric conversion device wherein hydrogenated amorphous silicon is employed as a photoconductor, and to the photoelectric conversion device produced by this process.
2. Description of the Prior Art
Of photoelectric conversion devices converting optical signals into electrical signals, there is known an image sensor for reading an original in a facsimile apparatus and the like.
FIGS. 1(a) and 1(b) are sectional views showing conventional planar type image sensors. The image sensor 10 of FIG. 1(a) has a construction wherein opaque electrodes 12 and 13 being opposed to each other are disposed on a substrate 11 with a suitable spacing, and a photoconductive material 14 and a passivation film 15 are successively laminated on the electrodes 12 and 13. Light is applied to the image sensor 10 from the side of the substrate 11 as indicated by an arrow La.
On the other hand, an image sensor 20 shown in FIG. 1(b) has a construction wherein a photoconductor 22 is formed on a substrate 21, opaque electrodes 23 and 24 being opposed to each other are disposed on the photoconductor 22 with a suitable spacing, and these electrodes 23 and 24 are further covered with a passivation film 25. Light is applied to the image sensor 20 from the side of the passivation film 25 (as indicated by an arrow Lb), contrary to the case of the image sensor 10 in FIG. 1(a).
FIGS. 1(c) and 1(d) are sectional views showing conventional sandwich type image sensors. The image sensor 30 of FIG. 1(c) has a construction wherein a transparent lower electrode 32 is formed on a substrate 31, a photoconductor 33 and an opaque upper electrode 34 are successively formed on the lower electrode 32, and a passivation film 35 is applied thereon. Light is applied to the image sensor 30 from the side of the substrate 31 as indicated by an arrow Lc in FIG. 1(c).
An image sensor 40 shown in FIG. 1(d) has a construction wherein an opaque lower electrode 42 is formed on a substrate 41, a photoconductor 43 and a transparent upper electrode 44 are successively formed on the substrate 41 and lower electrode 42, and a passivation film 45 is further applied thereon. In this case, light is applied to the image sensor 40 from the side of the passivation film 45 as indicated by an arrow Ld.
The substrate 11 in the image sensor 10 to which light is applied is transparent so that the substrate 11 functions also as the passivation film. Since the reverse passivation film 15 may be opaque, material and film thickness of the passivation film 15 can be selected comparatively tolerantly.
The image sensor 30 of sandwich construction is also in the same situation as that of the image sensor 10.
On the other hand, the passivation films 25 and 45 must be transparent in the image seensors 20 and 40 because light is applied from the sides of the passivation films.
Material for such passivation films may be phosphorous glass (PSG) prepared by the CVD (Chemical Vapor Deposition) method, and silicon nitride or silicon oxide prepared by the plasma CVD method, besides polyimide organic film and the like. Of the above materials, the most preferable is silicon nitride which is hardly permeable to water or an alkali ion such as Na and also, is heat resistive.
However, both silicon nitride utilized for the passivation film and amorphous silicon employed for the photoconductor have high residual stress, so that the silicon nitride film is likely to peel from the amorphous silicon film at the contacting portion therebetween. Such peeling becomes a cause for deterioration in the current-voltage characteristics of the photoelectric conversion device.
When a hydrogenated amorphous silicon film is heated at a temperature higher than 400.degree. C., an elimination reaction of hydrogen from the silicon atom arises. For this reason, it is preferable that the hydrogenated amorphous silicon film not be heated at a temperature higher than 400.degree. C. in the process of forming the passivation film. Accordingly, the most preferable method is the plasma CVD method in which low-temperature process treatment can be effected. The plasma CVD method, however, when it is intended for forming a silicon nitride film directly on a photoelectric conversion element, has the disadvantage that an ITO (indium-tin oxide) film changes in quality in the early stage of forming the silicon nitride film, and the characteristics of the junction between amorphous silicon and the ITO film deteriorate so that the dark current increases greatly.